More wierdness!
I corrected the slightly advanced timing as shown in the photos above, by retarding the whole plate, since both cylinders were showing nealy the same advance.
Restested and the horizontal is spot on , but the vertical is still the same state of advance, ie appears not to have changed.
Unsurprisingly, because Ive done nothing to change it, the strobe remains non functioning above idle on the vertical, and the vertical plug remains wet.